volcanic complex which occupies a surface of about approximately north of Galán, and which includes the large calderas of La Pacana, Cerro Guacha, Pastos Grandes and Cerro Panizos as well as more recent geothermal systems. This volcanism appears to be a surface expression of a pluton, and at depths of beneath the Altiplano-Puna volcanic complex electrical, gravity and seismic tomography data have localized a structure of partially molten rock called the "Altiplano Puna Magma Body". Volcanism in this "back" region may not be directly related to subduction processes despite the region itself being close to a subducting margin. The Galán caldera lies on the eastern margin of the Andes, where the Sierras Pampeanas begin. The region is characterized by the Puna, a high plateau similar to Tibet in Asia. Local Galán is a caldera with topographic dimensions of , of which about are part of the caldera proper. Such dimensions make Galán one of the biggest calderas on Earth. The floor of the caldera reaches an elevation of or about , and the whole caldera has an elliptical shape extending in the north–south direction. Only the western margin of the caldera structure appears to be a true caldera margin, however, with different landforms forming the rest of the caldera walls and the actual collapse caldera covering only a portion of the topographic caldera expression; the latter has been defined to be a volcano-tectonic depression. The caldera contains a resurgent dome, whose highest point in the frost-shattered Galán massif reaches an elevation of about -. Seismic tomography has identified a slow-speed anomaly beneath Galán, which has a volume of about and is considered to be a magma reservoir of the volcano. Summits along the caldera margin include Cerro Aguas Calientes to the north, Cerro Leon Muerto to the southeast, Cerro Pabellon to the southwest and Cerro Toconquis to the northwest. On the western rim, elevations of are reached. Younger volcanoes have developed on the western and northern rim of the Galán caldera. Hydrology The caldera contains a lake in its southwestern corner, which is known as Laguna Diamante. Laguna Diamante has gained attention among scientists for the extreme environmental conditions that life within the lake has to withstand, including high arsenic contents of the waters and high insolation with ultraviolet radiation. The water is hyper-alkaline and five times as salty as the sea but supports microorganisms which form microbial mats and